Weighting Principles?

[bkotheimer]

I think most experienced divers agree that the overall goal in adding weight is to achieve neutral buoyancy at the safety stop at the end of the dive with an empty BCD and 500 PSI in the tank.

So the ideal situation is to find yourself at 15' with 500 psi in your tank, an empty BC, a patient buddy, something or someone to hold excess lead as you add it to or subtract it from your rig. Then you can dial in your weighting. But we seldom find ourselves in exactly that situation. So we do the surface checks, the math based on our weight and wetsuit type, tank type, etc. so that we can be in the ballpark when we get to that safety stop.

I usually dive in a 7mm Henderson here in So.Cal. with a 7/5 hood, 5 mil gloves and boots. In addition to lead, my ballast includes a stainless steel DSS backplate and Apollo Bio-fins (which are -5# in saltwater, believe it or not). My tank is an AL80 (usually). The suit is new, very buoyant. I addressed this whole scenario in a recent thread where I determined that I can probably drop about 4-5# and will try so on my next dive.

However, I'm trying to plan weighting for various scenarios in more temperate and tropical locations and also the possible addition of a pony bottle. So I want to establish some guiding principles for my own weighting under different conditions. Without getting into particulars, here are some of the issues/practices where I'm curious about the opinions/experience of others on this very large forum...

1. When doing a weight check at the surface, many have advised - based on an AL80 - to add 5# after getting to float at eye level (with 1/2 full lungs, empty BC, full tank) to account for the air that will be consumed. Others, however, feel that the compression of the suit at the 15' safety stop and its resultant loss in buoyancy is about equal to the air consumed, making it unnecessary to add the 5#. This seems to be the opinion of Tobin over at DSS, who I consider to be somewhat of an expert on buoyancy and weighting.

So what do most people think? Add the extra 5 lbs, or not?


2. While I think the latter approach makes a lot of sense for a 7mm full wetsuit, there must be cases where a thinner suit would still require a few lbs be added after the surface check because it's going to lose less lbs of buoyancy. For instance, maybe you do add, say, 3 lbs if you're floating at eye level in a 5mm with an AL80... and 5 lbs if it's a 3mm. It would be nice to have a standard reference for figuring this out.

By any chance, is there a PERCENTAGE of its overall buoyancy that most neoprene loses at n number of feet?


3. For that matter, in figuring the buoyancy of a suit at the surface, why isn't there (or is there) a chart somewhere that says on the average a 4/3 men's medium is x lbs, 7mm men's large is y, etc.?


4. Tank/tub testing is going to require freshwater to saltwater conversion -- or is it?

So you hear advice sometimes to just throw all your neoprene in a tub of water and weight it until it sinks, and that's the weight you need. But for practical reasons, these kind of home tests are usually in fresh water. And the freshwater-->saltwater conversion scenarios I've seen are not just based on a simple percentage increase to your ballast, unfortunately, but are based on adding 2.5% of your weight plus your rig's weight to the amount of weight you used in fresh water. Crazy. When I determine my weighting needs based on the buoyancy of my suit and tank only, I don't consider my own weight, so why would I use my own weight as a factor when converting from fresh to saltwater?

Any chance that the aforementioned compression of your suit at the safety stop matches up with the weight difference between fresh and salt, such that if I need 20 lbs to hold my wetsuit under in a 2' deep freshwater tub, I also need about the same amount to hold my compressed wetsuit at 15' in saltwater? Wouldn't that be nice? Unlikely, but nice.


5. I'm adding a pony bottle to my rig. This isn't a post about pony bottles, so don't go there. My pony is an AL13 with a Zeagle razor and it's just over -3# full (and just over -2# empty). It's a bailout bottle only, so the air in it is for emergency use only and it's permanently attached to my rig. That makes it dead static weight, doesn't it? The same as if I'd put 3 lbs of lead on my cam band? After all, if I have to use it and I suck it down to empty, I'm going up anyway. So most of the time it's part of my ballast. Right?


So these are the questions/issues I'm curious about as I try to develop best practices for weighting myself under different conditions. I know that some, if not all, of these have been dealt with in various SB posts before, but oftentimes these are specific situations and general principles are hard to find. I've even checked out some of the sticky buoyancy calculators and wing lift calculators on this board, but they're confusing, contain errors in calculations and make assumptions that may not be true (I don't need 10# of lift for my head weight -- what the hell is head weight?) That said, kudos to the authors for the attempt.

-bk

 

[DiveGear4Less]

First, let's step back and look at your first statement. You are incorrect. The perfect weighting is to be neutral at the surface with 500 psi in your tank. Remember, if you weight for your 15 foot stop and only having 500 psi, you still have to ascend that last 15 feet. If you are neutral at 15 fsw, you will be positive at 14 fsw. Therefore your ascent will be on the uncontrolled side. Roughly 700-800psi is equivalent to 1 lb. So add about 4 lbs to your full tank at the surface weight check and you should be okay at the end of the dive.

As for figuring out buoyancy characteristics of suits, it's pretty much impossible. All suits are made differently and have a different amount of neoprene. Also, after you dive a suit for a while, it loses some of its thickness and becomes less buoyant over time. There's no single formula to account for all the variables.

The pony bottle is dead weight, but it will not weigh as much under water as it does on the surface. You need to figure out its negative buoyancy - dry weight minus water displacement.

 

[vladimir]

But for practical reasons, these kind of home tests are usually in fresh water. And the freshwater-->saltwater conversion scenarios I've seen are not just based on a simple percentage increase to your ballast, unfortunately, but are based on adding 2.5% of your weight plus your rig's weight to the amount of weight you used in fresh water. Crazy. When I determine my weighting needs based on the buoyancy of my suit and tank only, I don't consider my own weight, so why would I use my own weight as a factor when converting from fresh to saltwater?

Because the upward force--buoyancy--that you are trying to counteract with lead is equal to the weight of the water that you displace (Archimedes principle). You're not really concerned with your weight, but with the weight of the water you displace. Since the specific density of fresh water is 1, they are equal, but it's easier to weigh you and your kit than the water. You will be displacing the same volume of water whether it is fresh or salt, but the saltwater will weigh more--about 2.5% more.

 

[dumpsterDiver]

Personally, I would want to be able to remain slightly negative at the end of the dive with zero air in my tank.. this would allow me to control my bouynacy and to also not be forced to float up from say 10 ft and risk getting run over by a boat. Also, in order to deploy and hang under an SMB, the diver needs to be negative so that they can pull down on the smb and keep it errect.

 

[SkimFisher]

I think most experienced divers agree that the overall goal in adding weight is to achieve neutral buoyancy at the safety stop at the end of the dive with an empty BCD and 500 PSI in the tank.

Huh? Never heard that part before. I prefer to be slightly negative as well depending on the diving environment. Actually, the ability to GO negative if I need to is what I'm looking for.

 

[fnfalman]

The purposes of proper weighting, at least to my mind, are of twofolds:

1. To be able to execute a safety stop with ease. That doesn't mean being neutral at 15-ft depth, but it means being able to maintain your safety stop via the simple act of breathing in and out. No air in the wing, tank fairly empty, and you can hover at safety stop depth just by regulating your breathing.

2. To me, this is more important than No. 1, is to be able to dive without having to add a lot of air to the BC. When you're overweighted, you're constantly putting in air and getting it out of your BC. To be able to descend in a slow, controlled pace. If you need to halt at 5-ft because your ears can't equalize, then you can halt at 5-ft. If you need to pause at 10-ft and check on your buddy to make sure that he/she still follows you down, then you can do that with ease. If you're overweighted and you hit the deflate button, you're taking the express elevator ride down to the bottom. When you get close to your desired depth, it doesn't take much to establish buoyancy.

 

[GrumpyOldGuy]

There are subtle differences in the perfect weight for divers in dry suits vs thin wetsuits vs thick wet suites. There is no universal rule, if there was we would not be arguing about such a seemingly simple thing again and again.

Even such a simple statement as neutral at surface with 500psi is ambiguous, is this before the dive or after the dive (w/s is crushed), full lung or empty?

In the end, I want the minimum weight to hold my stop, ascend very slowly and be able to dive down to avoid a boat. I know what this feels like based on experience, but to make a set of rules that would apply to all other divers is incredibly hard. In the end, you need to practice all 3 skills at 500PSI and you will know if you have enough weight.

 

[vladimir]

2. While I think the latter approach makes a lot of sense for a 7mm full wetsuit, there must be cases where a thinner suit would still require a few lbs be added after the surface check because it's going to lose less lbs of buoyancy. For instance, maybe you do add, say, 3 lbs if you're floating at eye level in a 5mm with an AL80... and 5 lbs if it's a 3mm. It would be nice to have a standard reference for figuring this out.

By any chance, is there a PERCENTAGE of its overall buoyancy that most neoprene loses at n number of feet?

You wouldn't expect it to be linear, would you? If you take a balloon down to 10 meters it loses half it's buoyancy, right? And at 20 meters it has lost 2/3, right? I wouldn't expect neoprene to behave like a balloon exactly, either, but it wouldn't be linear.

3. For that matter, in figuring the buoyancy of a suit at the surface, why isn't there (or is there) a chart somewhere that says on the average a 4/3 men's medium is x lbs, 7mm men's large is y, etc.?;

Here's the first one that turned up in a google search:

Buoyancy & temperature

I'm sure there are others, but I wouldn't rely on them; there's too much variation from brand to brand and even between a new suit and an older suit for them to be more than a rough guide.

 

[Peter Guy]

This is one where close is good enough. Don't forget, every time you change your gear you will have some change in your overall displacement and thus the "perfect" weighting will change -- not to mention the age/condition of your wetsuit, etc. (And let's not even begin to talk about changes in the salinity of the water which is a significant issue where I generally dive, Puget Sound.)

The simple key is that you need to have enough lead to keep you where you want to be in the water column and for me that means I will try to err on the side of being negative a pound or two or three -- depending on how cold the water is since I dive dry. I can always add air (positive buoyancy) to my wing or drysuit to counter being negative -- but it is hard to subtract positive buoyancy if that is what is needed while in the water column.

 

[TSandM]

bkotheimer, you'll get different answers from different people in part because people prioritize things differently.

The biggest thing you need from your weight is to be able to stay underwater at a depth you choose (in other words, control an ascent). Being too light ranges from uncomfortable to dangerous. Therefore, it is my position that, if you are going to err on your weighting, always do it in the direction of being overweighted.

The second thing about weighting is that extra weight requires extra gas in the BC, which makes buoyancy control harder. Therefore, you don't want to carry a ton more weight than you need, because it makes your life difficult.

The third thing is that, as you gain facility in diving, that gas in the BC or dry suit becomes easier to manage. This swings the balance back toward preferring overweighted to underweighted, and in fact, many of us in dry suits consciously carry more weight than we actually NEED by the parameters you describe, in order to run more air in the suit and stay warmer.

The differences between weighting for neutral at the surface with 500 psi, and weighting at 15 feet with 500 psi, and weighting at the beginning of the dive versus the end of the dive, are differences that have to do with absolute fine-tuning of weight. My personal experience is that it's worth playing with this in warm water with very minimal exposure protection, because if you get your weight absolutely minimized for the functional capacity you want (note the comments about being able to keep an SMB vertical above, which are quite true) then diving becomes a thing of real nuance with your breathing, and that's fun.

When you do the weight check at the surface with a full tank, the weight you add doesn't have anything to do with your exposure protection. It has to do with the gas in the tank that you intend to use and exhaust into the water. That gas has weight, so you get lighter as it's used, and you have to carry lead to make up for this. Therefore, the amount of weight you add will always be the same, for the same tank. With an Al80, that is reasonably 5 lbs. (Air weighs approximately one pound for 13 cubic feet.)

Where the buoyancy loss of your wetsuit becomes important is in calculating the lift you need in your BC, because the buoyancy the suit loses at depth is buoyancy you have to add elsewhere. I think the reason there is no table of how much you can lose is because it's quite variable with size of person and brand or type of neoprene. It's not quite as simple as compression of gas, because the neoprene itself has some resistance to crushing. And this resistance to changing form also, apparently, operates in reverse -- so the suit doesn't rebound to its full buoyancy at the end of the dive. This is why, when the suits are thick, you can carry a little less weight than you would otherwise calculate. I'm not sure how well that works, since I have never dived a thick wetsuit, but Tobin says it does, and he HAS, and I trust him.

So, in short: Setting your weighting by weighting yourself neutral at the surface with a full tank and adding a pound for every 13 cubic feet of gas you intend to use during the dive will get you very close to your ideal weight. Ideal weight varies according to your priorities. Light is worse than a little bit heavy. Buoyancy loss from exposure protection determines your required lift, not your weighting.

Hope that helps.